Natural gas is a clean-burning, relatively low priced alternative to gasoline and diesel as a transportation fuel, particularly in fleet and industrial vehicles. Natural gas is an odorless, nontoxic, gaseous mixture of hydrocarbons (primarily of methane—CH4), but may also contain ethane, propane and heavier hydrocarbons. Because of the gaseous nature of natural gas, it is generally stored onboard a vehicle in either a compressed natural gas (CNG) state where pressurization may be about 3,000 psi, or a liquid natural gas (LNG) state, in thermally insulated tanks.
As used herein, the term “liquid natural gas fuel” may include natural gas, methane, ethane, ethylene, and/or hydrogen. It may also be referred to as liquefied cryogenic fuel, cryogenic LNG, or LNG. This liquid natural gas is stored at a certain temperature and pressure in a vehicle's tank. While the detailed description describes natural gas examples, it should be understood that other cryogenic fuels may be used as well.
LNG is produced by purifying natural gas and super-cooling it to −260° F., condensing it into a liquid, and reducing its volume to about 1/600 of its original volume. This denser form (as compared to CNG) has more potential energy for the amount of space it takes up, and more energy can be stored in the same amount of space on a vehicle.
LNG vehicle fuel provides an excellent means to reduce emissions. Due to the clean-burning nature of natural gas, LNG powered heavy-duty vehicles can achieve low emission rates without excessive and expensive emission control equipment as is required for diesel engines. One drawback of natural gas vehicles (NGVs) is their limited driving range and the absence of current infrastructure to re-fuel LNG vehicles.
Thus, fuel-intensive fleets are better-suited to run on natural gas, including LNG. A larger volume of vehicles makes it more economical to build natural gas engines, and fleets are more able to install their own natural gas infrastructure.
Large mining trucks are often operated in sub-freezing temperatures. At such low temperatures, a problem arises in converting, or vaporizing, the LNG to a gas form. While LNG has a low boiling temperature, the LNG may be at such a low temperature that contact with a vaporizer (often liquid), even indirect contact, may freeze the vaporizer.
Some engines may operate on both natural gas and diesel, known as twin or bi-fuel vehicles. The operator may switch between fuel types, with natural gas being the economical fuel of choice. Twin fuel trucks vaporize cryogenic LNG to burn in its engine. Engine coolant may be used for this. During initial engine startup, the coolant may be at a low temperature, and the coolant may freeze when vaporizing the LNG, blocking flow of coolant in the conduit.
Prior solutions include waiting for the engine to warm up and/or running the engine for a longer period of time using diesel fuel. After the coolant warms up in the engine, the LNG pump is switched on. Warming the engine up enough to warm the coolant may take 10-20 minutes or as long as several hours. Another approach is to include a large accumulated volume of vaporized (ready-to-combust) natural gas at a high pressure, separate from the liquefied state of the natural gas, which may run the engine while the engine warms up. However, large high pressure tanks are very expensive and take up a lot of vehicle space.
U.S. Pat. No. 4,231,226 to Griepentrog presents a solution to vaporize liquid natural gas by directing it into a heat exchange relationship with waste heat produced from a thermal power engine. Vapor discharged from the turbine is passed, and liquid natural gas is moved, through a heat exchanger, to cool the vapor and evaporate the liquid natural gas. In the '226 patent, exhaust gas is used as the vaporizer.